demandimport: drop hack for old Pythons which had no level argument
_import() was added by
868282fa29d8, but it is identical to _origimport()
now.
color: drop the now useless color extension
all the extension features are provided by core since 4.2.
extensions: expand the builtins extensions declaration
This will future updates of the set cleaner and more readable.
debugextensions: drop the color from the test
We are about to remove the extension so we remove trace of it in this specific
test to prevent it to be confused.